The present invention relates to a transparent conductive film used in a liquid crystal display device or an electroluminescence (EL) display device.
2. Description of the Related Art
Conventionally, a transparent conductive film made of Indium Tin Oxide (ITO) has been widely used as a transparent conductive electrode for a liquid crystal display device or an EL display device, because such a transparent conductive film has various advantages in that it has low specific resistance and high visible light transmittance, and it can be subject to a wet etching step with an aqueous solution, etc.
FIG. 2 exemplarily shows an active matrix substrate using a transparent conductive film made of ITO as a pixel electrode. In this active matrix substrate, a plurality of pixel electrodes 106 are arranged in a matrix on a glass substrate. The pixel electrodes 106 are each connected to a thin film transistor (TFT). The TFT is used for a switching function to switch an electrical field applied between the respective pixel electrodes 106 and respective counter electrodes (not shown), and thus a display pattern is formed.
The process for fabricating such a conventional active matrix substrate will be described. First, a base insulating film 100 made of Ta.sub.2 O.sub.5 is formed so as to entirely cover the substrate by sputtering. Next, a lower gate wiring 101 and an upper gate wiring 102 are formed on the base insulating film 100 by sputtering. The lower gate wiring is made of aluminum (Al) so as to reduce the resistance thereof. The upper gate wiring 102 may be made of a metal such as tantalum (Ta) which is capable of being anodized. Thereafter, the surface of the upper gate wiring 102 is anodized in electrolyte, and thus an anodic oxide film made of TaO.sub.x is formed. Then, a gate insulating film 107 made of SiN.sub.x is formed so as to entirely cover the substrate by a CVD method. The next step is that an intrinsic amorphous silicon semiconductor layer is formed on the anodic oxide film, and then a SiN.sub.x layer, which functions as an etching stopper 103, is formed thereon. In addition, an amorphous silicon of an n.sup.+ type semiconductor layer 104 doped with P (Phosphorus) is formed so as to cover an edge portion of the etching stopper 103 and the semiconductor layer. Subsequently, a source/drain wiring 105 made of titanium (Ti) is formed by sputtering. The source/drain wiring 105 is made as a single layer. Thereafter, a transparent conductive film made of ITO is formed by sputtering, and then this film is patterned into the pixel electrode 106. Furthermore, a protective film 108 is formed on the transparent conductive film.
The above transparent conductive film is generally formed as follows: First, in a sputter chamber, a target made of indium oxide and tin oxide is arranged so as to face the substrate on which the transparent conductive film will be formed. Then the substrate is heated up to a temperature of around 280.degree. C. Thereafter, the air in the sputter chamber is discharged so as to create a high vacuum atmosphere, and then a sputter gas made by adding several vl % (volume %) of gaseous oxygen to an argon gas is supplied to the sputter chamber at a prescribed gas pressure. Thereafter, an electric discharge is done between a substrate holder to hold the substrate and the target, and thus the sputtering process is performed. Finally, an ITO film is formed on the substrate.
The carrier density of the thus formed ITO film is reduced as the oxygen partial pressure of the ITO film is increased, or the oxygen deficiency of the ITO film is lowered. The reason is that the oxygen deficiency of the ITO film functions as a donor. On the other hand, the carrier mobility is increased as the oxygen deficiency of the ITO film is decreased. The specific resistance of the ITO film is lowered as the carrier density of the ITO film is increased, or the carrier mobility of the ITO film is increased. Therefore, in the case where the rate of the oxygen parts shows a prescribed value, the specific resistance of the ITO film shows a minimum value. The light transmittance of the ITO film is increased as the oxygen deficiency of the ITO film is decreased. On the other hand, the light transmittance of the ITO film is limited by the degree of the oxygen deficiency of the ITO film when the specific resistance of the ITO film shows the minimum value. Therefore, according to the above-mentioned method, the amount of the gaseous oxygen to be included in the sputter gas is adjusted so as to obtain an ITO film having a desired amount of oxygen.
However, the above-mentioned method has a problem in that it is difficult to control the wet etching step for forming a wiring pattern at a later stage, because the ITO film is formed with less oxygen deficiency due to the gaseous oxygen added to the sputter gas during the ITO film forming process.